Metal Halide Ballasts
Although the title says metal halide ballasts, these principles can be applied to any type of grow lighting. All high intensity discharge (HID) grow lights, whether metal halide or high pressure sodium (HPS), require a ballast to ignite and maintain the bulb's activity. Some fluorescent bulbs also require a ballast, depending on the type (usually the tube lights).
It is very important that you choose the correct electronic ballast for your grow lights. Metal halide bulbs include an ignitor at their base, while high pressure sodium (HPS) bulbs do not. Also, the ballast must be designed for the correct wattage of the HID bulb or it may burn out or even catch fire if it shorts out. Conversion ballasts are available that are capable of switching between metal halide and HPS bulbs, usually by tunring off the ignitor.
Electronic ballasts can be used with fluorescents and lower wattage HID bulbs (in the 35-100 watt range) to save on electricity usage. Many growers report energy savings of 10-20%, just by switching their grow lights to an electronic ballast setup. Other advantages of electronic ballasts include reduced flicker and lighter weight when compared to their magnetic counterparts.